JP2000330441A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To appropriately set the press-contact force of a cleaning blade without shortening the lives of a toner image carrier such as a photoreceptor drum and the cleaning blade by specifying the press-contact force of the cleaning blade. SOLUTION: This device is equipped with a cleaning blade 73 whose edge is pressed by a press-contact force (f) toward the surface of the toner image carrier. The press-contact force (f) (unit: g/mm) of the blade 73 is defined as force applied per the unit length of the blade 73 and satisfies fmin+S / 6×D×(1-S)}<=f<=fmax. The symbol S stands for circularity and S = (the peripheral length of the projected image of a toner particle) and satisfies 0.95<=S<=1, D is the average particle size (unit: μm) of toner to be used, fmin (unit: g/mm) is minimum press-contact force and satisfies 1<=fmin, and fmax (unit: g/mm) is maximum press-contact force and satisfies 5<=fmax<=10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine and a printer.

[0002]

2. Description of the Related Art An electrophotographic image forming apparatus, for example,
2. Description of the Related Art In electrophotographic copying machines and printers, fine toner particles are used to form a visible image on paper.
This toner has conventionally been manufactured by pulverizing raw materials. Because of the pulverization, individual toner particles ranged in shape from nearly spherical to distorted shapes far from spherical, and their diameters ranged from large to small. .

[0003] In recent years, the development of a production method by polymerization has promoted further reduction of the particle size and spheroidization of toner.
The image quality and transferability of the formed image have been greatly improved.

In an electrophotographic image forming apparatus,
The toner image on the photosensitive drum is directly transferred to paper, or is temporarily transferred from the photosensitive drum to a transfer drum or a transfer belt, and then transferred to paper. After the transfer of the toner image, the toner image carrier (photosensitive drum, transfer drum or transfer belt) is cleaned to remove a small amount of residual toner in preparation for the next image formation.

In many cases, this cleaning is performed by pressing a cleaning blade, which is obtained by cutting polyurethane rubber into a blade shape, with a suitable force against the surface of the toner image carrier that moves cyclically.

The reduction in the particle size of the toner causes an increase in van der Waals force on the toner image carrier. On the other hand, spheroidization of the toner significantly enhances the rolling properties of the toner.

[0007] Because these effects are doubly affected, it is becoming increasingly difficult to clean the formed image with a cleaning blade in order to improve the quality of the formed image and improve the transferability.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a cleaning unit (cleaning device) having a cleaning blade or an image forming apparatus. Even if the spheroidization is further promoted, the pressure of the cleaning blade is excessively increased, and as a result, the life of the toner image carrier such as the photosensitive drum and the cleaning blade is not shortened. It is an object to make it possible to set

[0009]

The above-mentioned object is attained by the invention having the following constitution.

An electrophotographic image forming apparatus, a cleaning apparatus for use in the image forming apparatus, or a cleaning method in the cleaning apparatus,
The cleaning device includes at least one cleaning blade whose edge is pressed by a pressing force f toward the surface of the toner image carrier in order to clean the surface of the toner image carrier provided in the image forming apparatus. And a pressing force f (unit: g) of the cleaning blade.
/ Mm) is a force applied per unit length of the cleaning blade, and fmin + S / {6 × D × (1-S)} ≦ f ≦ fm
ax.

Here, S is a circularity, and S
= (Perimeter of a circle having an area equal to the area of the projected image of the toner particles) / (perimeter of the projected image of the toner particles), and 0.95 ≦ S ≦ 1, where D is the toner used Is the average particle diameter (unit: μm) of the above fm
in (unit: g / mm) is the minimum pressing force, and 1 ≦
fmin Also, the above fmax (unit: g / mm) is the maximum pressing force, and satisfies 5 ≦ fmax ≦ 10.

The electrophotographic image forming apparatus, the cleaning apparatus used in the image forming apparatus, or the cleaning method in the cleaning apparatus may further have the following configuration.

The value of the minimum pressing force fmin is substantially 2 g / mm.

The value of the maximum pressing force fmax is substantially 5 g / mm.

The cleaning blade is pressed against the toner image carrier in the counter direction.

The material of the cleaning blade is polyurethane rubber.

The cleaning blade is pressed against the toner image carrier at a pressure contact angle of 3 to 25 degrees.

The cleaning blade is pressed against the toner image carrier at a pressure contact angle of 5 to 15 degrees.

The free end length of the cleaning blade is 3
The length is in the range of 20 mm to 20 mm.

The free end length of the cleaning blade is 5
mm to 15 mm.

The cleaning device includes a second cleaning member in addition to the cleaning blade, and the second cleaning member is any one of a cleaning brush, a cleaning roller, and a cleaning blade.

The toner image carrier is any one of a photosensitive drum, a transfer belt and a transfer roller.

[0023]

FIG. 1 is a sectional view schematically illustrating a cleaning unit and an image forming apparatus having the same according to an embodiment of the present invention.

In a widely used electrophotographic image forming apparatus, a charging unit 2, an exposure unit 3, a developing unit 4 and a transfer unit 5 are arranged around a photosensitive drum 1 rotating in the direction of an arrow. .

The photosensitive drum 1 has a cylindrical portion whose surface is made of a photosensitive material. The charging unit 2 sequentially and uniformly charges the surface of a portion passing therethrough by the rotation of the photosensitive drum 1.

The exposure unit 3 irradiates the surface of the photosensitive drum 1 with light according to the density of the image to be formed. The exposure unit 3 has a different configuration between an analog image forming apparatus and a digital image forming apparatus.

In an analog image forming apparatus (analog copying machine), a document placed on a document table (not shown) is irradiated with light, and the light reflected by the document is reflected by some lenses and some reflecting mirrors. As a result, it is guided to the surface of the photosensitive drum 1 and forms an image thereon.

In a digital image forming apparatus, for example, a digital copying machine or a printer, a laser, LE
D or others are used.

In the type using a laser, the laser beam scans the surface of the photosensitive drum 1 linearly along its axial direction. In the type using LEDs, the number of LEDs corresponding to the pixels is provided, and each LE
D irradiates each point on the line of the photosensitive drum 1 with light.

When the photosensitive drum 1 is irradiated with light, the photosensitive layer on the surface becomes conductive, and the charged charge flows toward the conductive base supporting this layer of the photosensitive drum 1, and Charge is lost. As a result, a difference in charged state occurs between a portion irradiated with light and a portion not irradiated with light.

When the charged toner is supplied to the surface of the photosensitive drum 1 in such a state by the developing unit 4, a portion where the toner particles are attracted according to the charged state of the photosensitive drum 1, The development is performed by the occurrence of a part that is not performed.

The transfer unit 5 transfers the developed toner image to a sheet. Further, depending on the type of the image forming apparatus, the toner image is temporarily transferred to a transfer drum or a transfer belt, and then transferred to a sheet by another transfer unit.

In all the transfer steps, not all the toner particles move to the paper, the transfer drum or the transfer belt, so that a small amount of toner is exposed even after passing through the transfer unit 4 (or another transfer unit). It remains on the surface of the body drum 1 (or the transfer drum or the transfer belt).

In the following, an example of an image forming apparatus in which a toner image on the photosensitive drum 1 is directly transferred to a sheet will be described. However, the image forming apparatus may be mounted on another toner image carrier (the above-described transfer drum or transfer belt). In the image forming apparatus of the type in which the toner image is transferred onto a sheet, the following cleaning problem commonly occurs.

In the next step, the surface of the photosensitive drum 1 is cleaned by the cleaning unit 7.

The static elimination lamp 8 includes a cleaning unit 7
After the residual toner is removed, the charged state slightly remaining on the surface of the photosensitive drum 1 is removed. The portion of the photosensitive drum 1 that has passed in front of the charge removing lamp 8 is charged again by the charging unit 2, and the above-described process is repeated.

The cleaning unit 7 includes a cleaning blade 73 and another cleaning means (for example, a cleaning brush 75 or a cleaning roll) or two cleaning blades in addition to the type having the only cleaning blade 73 as described above. Some types are used together.

The present invention is applicable to any type of cleaning unit as long as a cleaning blade is used.

The cleaning blade 73 is oriented in a direction (counter direction) facing the rotation direction of the photosensitive drum 1 (indicated by an arrow in FIG. 1).

A large number of sheets are stored in the sheet cassette 11. Sheets are taken out from the sheet cassette 11 in order from the top, and each sheet is conveyed by a conveying roller and / or a conveying belt and supplied between the photosensitive drum 1 and the transfer unit 5.

While passing between the photosensitive drum 1 and the transfer unit 5, the toner adsorbed on the photosensitive drum 1 is transferred onto a sheet.

The transfer charger 51 of the transfer unit 5 moves the toner adsorbed on the photosensitive drum 1 onto a sheet by electrostatic force. At this time, since the sheet is strongly attracted to the photosensitive drum 1 by the electrostatic force, the separation charger 52 neutralizes the static electricity of the sheet so that the sheet can be easily separated from the photosensitive drum 1.

A DC voltage is applied to the transfer charger 51, and an AC voltage is generally applied to the separation charger 52.

The sheet on which the toner is adsorbed is conveyed to the fixing unit 13 by a conveying roller and / or a conveying belt, and the toner is fixed on the sheet by the pressure and heat applied by the fixing unit 13.

The paper on which the toner has been fixed is transported by a transport roller and / or a transport belt, and is stacked on a paper discharge tray. The user can pick up the paper on which copying or printing has been completed from the discharge tray 14.

FIG. 6 is a partially enlarged view of the cleaning unit 7. The cleaning unit 7 has a cleaning blade 73 made of polyurethane rubber, and removes residual toner by pressing the edge of the cleaning blade 73 against the photosensitive drum 1.

The polyurethane rubber used here is:
It is produced by reacting an isocyanate and a polyol, and there are ester-based polyols and ether-based polyols, and either of them may be used.

A blade holder (holding member) 71 for holding the cleaning blade 73 is rotatably supported by a rotation fulcrum 70, and a force point portion provided at a position opposite to the cleaning blade 73 with respect to the rotation fulcrum 70 by a spring. 72
The cleaning blade 73 is pulled by the tension of
Are pressed against the photosensitive drum 1 (constant pressure load method).

FIG. 2 is a graph showing the particle size distribution of general toner particles and explaining the volume average particle size. In this graph, the horizontal axis represents the particle diameter on a logarithmic scale, and the vertical axis represents the frequency of appearance of toner particles having that particle diameter. The particle size distribution of general toner particles is substantially a normal distribution as shown by the solid line in FIG.

Further, when particles having a particle diameter equal to or larger than a certain particle diameter are integrated and a ratio to the total number of toner particles is obtained,
A line indicated by “integration ratio” in FIG. 2 is obtained. The particle diameter at which the integration ratio becomes 50% is the average diameter (D50).

The average diameter includes a number average diameter and a volume average diameter. The number average diameter is an average diameter of only the number, and the volume average diameter is a weighted average in consideration of the particle volume. The description of the present invention is made based on the latter volume average diameter.

FIG. 3 is an explanatory diagram for explaining the circularity used in this specification.

When a projected image of an arbitrary particle is considered, the area of the projected image is defined as A1, and the peripheral length of the projected image is defined as L1. Consider a circle having an area equal to this area A1, and define its circumference as L
Let it be 2. The degree of circularity (S) is the ratio of L2 and L1 (L2 / L
Defined by 1). Since the circle is a closed curve having the shortest perimeter per area, L1 ≧ L2, that is, 0 ≦ S = L2 / L1 ≦ 1.

Since the projected image of the particle has a different shape depending on the projection direction (that is, depending on the attitude at which the particle is observed), the circularity also has a different value correspondingly. However, a sufficiently large number of particles, e.g.
When 000 particles are observed, the influence of the projection direction is averaged. Therefore, it can be said that the circularity (S) obtained as described above generally represents the characteristics of the particle group.

FIG. 4 shows that the volume average diameters were 6 μm, 7 μm and 8 μm.
m, and their average circularity is 0.95, 0.9
9 is a graph showing the results of an experiment in which the cleaning property of a cleaning blade was evaluated by using a total of 9 toners of 8, 0.99 as samples.

Each plot point in this graph indicates the pressing force when cleaning was possible in practical use in consideration of durability performance by the cleaning blade. Here, the pressing force is obtained by calculating a component force of the pressing force of the edge of the cleaning blade 73 contacting the photosensitive drum 1 toward the center of the photosensitive rest drum 1 per unit length of the cleaning blade 73. . Normally, the pressing force is set to be about 1 g / mm higher than the theoretical value in consideration of durability performance.

That is, a value obtained by dividing the total load of the photosensitive drum 1 in the center direction by the length of the cleaning blade 73 (also referred to as a linear pressure, the unit is [g / mm]).

The cleaning performance was evaluated as follows. That is, in a low temperature and low humidity environment, the diameter φ1
0.8 mg / cm ^{2 on} the surface of the photosensitive drum 1
And the photosensitive drum 1 was caused to make one revolution. At this time, the photosensitive drum 1 was stopped immediately after passing through the cleaning blade 73, and the determination was made based on the presence or absence of toner that passed through the cleaning blade 73 (that is, was not removed).

The closer the plot point is to the right, that is, the higher the pressure contact side, the worse the cleaning performance. As the toner has a smaller diameter or is more spherical (that is, the circularity S is higher), the plot points are distributed on the high pressure contact side (right side), and it can be read from this graph that the cleaning property is poor.

Next, such a phenomenon will be considered.

A van der Waals force acts between the toner particles and the surface of the photosensitive drum 1.

The potential energy E of the Van der Waals force acting between two molecules is expressed as E = β / r ⁶ . When the sum of all the molecular forces constituting the object is calculated, the van der Waals force of the object is obtained. The van der Waals stress between two semi-infinite plates separated by a distance z is expressed by the following simple expression by volume integration.

Fv = A / (6πz ³ ) where A is a van der Waals constant. Also, when this equation is applied to particles, Fv = A · d / (12 · z ² ) d = D ₁ · D ₂ / (D ₁ + D ₂ ).

Here, D ₁ and D ₂ are the diameters of two adjacent particles. Further, Fv = A · D ₁ / (12 · z ² ) by setting F ₂ between the particle and the plane as D ₂ = ∞.

Further, the Fv between the particle having the unevenness on the surface and the plane is as follows: Fv = A · D ₁ / {12 · (z + z ′) ² }

Here, z ′ is the surface roughness of a plane (really,
0.01 μm). Now, considering the particles attached to the plane, z << z ', so that z can be ignored.

Therefore, the above equation is as follows: Fv = A · D ₁ / (12 · z ′ ² )

Further, by multiplying the above equation by the number of particles per unit area (Nt), Fv per unit area is calculated as follows: Fv = A · D ₁ / (12 · z ′ ² ) · Nt Can be sought.

Here, D _{1 is set} to D again, and z ′ is set again.
Is expressed as z, Fv = A · D / (12 · z ² ) · Nt (1)

Here, A is a van der Waals constant, A ≒ 1 × 10 ⁻¹⁹ [J: Joule], D is the volume average diameter of the toner [μm], and z is the separation distance Nt between the particle and the plane. Is the number of particles per unit area (mm ² ) of the surface of the photosensitive drum 1.

The smaller the particle size of the toner, the smaller the van der Waals force Fv [N: Newton] of one particle, but the larger the particle size per unit area of the photosensitive drum 1 becomes. As a result, the smaller the particle size, the larger the total Fv.

In practice, the surface roughness of the photosensitive drum 1 is 0.01
Since μm, the above z is z = 1 × 10 ⁻⁵ [mm], so Nt is Nt = (D · 10 ⁻³ ) ⁻² . ... (2)

By substituting these into equation (1), Fv = 10 ⁻³ / (12 · D) (3)

The ease of cleaning by the cleaning blade (cleaning force: F _CL ) is determined by the van der Waals force Fv of the toner and the rolling property. Here, the circularity ratio P is defined as a parameter representing the rolling property as P = S / (1-S) (4).

This equation shows that the more the toner particles are circular (that is, S
The value of P increases as the value approaches 1 from the 0 side (the value of P approaches infinity as the toner particles approach a perfect circle),
In other words, the larger the circularity ratio P is, the easier it is to roll.

On the other hand, when S approaches the value 0, P also approaches the value 0, that is, the shape of the toner particles largely deviates from the circular shape, indicating that the toner particles are hard to roll.

The cleaning force F _CL [N] is represented by F _CL = Fv · P (5)

[0078] The pressing force f necessary to remove the toner with _{F CL} [g / mm] _{is, f ≧ f min + α ·} F CL / G ··············・ (6)

Here, f _min is the minimum pressure contact force, and varies depending on the degree of setting of the life and the like. Usually, f _min ≧ 1, preferably f _min ≧ 2. G is a gravitational acceleration, that is, G = 9.8 × 10 ⁻³ [mm / s ² ].
(Lkg = 9.8 N, lg = 9.8 × 10 -3 N) is _necessary in order to convert the _{F CL} from [N] to [g].

Α is a conversion coefficient for converting a toner having a cleaning force _FCL into a pressure contact force for cleaning, and α ≒ 20 in correspondence with an experimental result. Therefore, if f _min = 1 and “1” in consideration of the durability performance is added and equation (6) is rewritten, f ≧ 1 + P / (6 · D) +1 f ≧ 1 + S / ｛6 · D · (1-S )｝ + 1 (7) FIG. 5 is a graph of Expression (7). It can be seen that the results almost coincide with FIG. 4 showing the experimental results.

If a pressing force larger than the pressing force f represented by the equation (7) is set, it can be seen that cleaning is possible.

However, if the pressing force is too high, side effects such as squealing of the cleaning blade and sharpness may occur. These are less related to the particle size and circularity of the toner, and are likely to occur in a state where toner is not supplied between the cleaning blade 73 and the photosensitive drum 1.

From the experience so far, the side effects described above gradually occur under the condition that the pressing force f of the cleaning blade 73 is 5 g / mm or more.
It can be used up to g / mm, and under the condition of 10 g / mm or more, this side effect occurs frequently, so that it cannot be used substantially.

Accordingly, when the pressing force f is 1 + S / {6 · D · (1-S)} ≦ f ≦ 10, more preferably, 2 + S / {6 · D · (1-S)} ≦ f ≦ 5 By setting the value in the range, the cleaning blade can exhibit desired cleaning performance, and it is possible to reliably prevent the cleaning blade from squeaking or causing a problem such as a sharpness.

FIG. 6 is an enlarged view of the cleaning means in the embodiment of the present invention. The cleaning blade is of a counter type for the toner image carrier such as the photosensitive drum 1 (the tip of the cleaning blade is
The free end length 1 of the blade, the thickness t of the blade, and the pressing angle φ are in the following ranges: the free end length 1 of the blade: 3 to 20 mm; preferably 5 to 15 mm. t: 0.5 to 5 mm; desirably 1.5 to 3 mm Pressing angle φ: 3 to 25 °; desirably 5 to 15 °.

As described above, a composite cleaning unit combining a cleaning blade and a second cleaning member (for example, another cleaning blade or a cleaning brush) or an image forming apparatus including the same is also applicable. The present invention can be applied.

[0087]

As described above, according to the present invention, in order to improve the image quality and the transferability of the formed image, the cleaning blade is excessively reduced even if the toner is further reduced in diameter and spheroidized. As a result, the life of the toner image carrier such as the photosensitive drum or the cleaning blade is not shortened, and furthermore, the cleaning blade can be properly cleaned without causing the cleaning blade to squeal or generate a crack. There is an effect that the pressing force can be set.

[Brief description of the drawings]

FIG. 1 is a sectional view schematically illustrating a cleaning unit of the present invention and an image forming apparatus including the same.

FIG. 2 is a graph showing a particle size distribution of toner particles and explaining a volume average particle size.

FIG. 3 is an explanatory diagram for explaining the definition of circularity used in the present specification.

FIG. 4 shows a volume average diameter of 6 μm, 7 μm, 8 μm,
Each average circularity is 0.95, 0.98, 0.99
9 is a graph showing experimental results obtained by evaluating the cleaning property of the cleaning blade by using a total of 9 toners as samples.

FIG. 5 is a graph plotting a minimum pressing force f obtained by Expression (7).

FIG. 6 is a partially enlarged view of the cleaning unit 7;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photoconductor drum 2 charging unit 3 exposure unit 4 developing unit 5 transfer unit 7 cleaning unit 8 static elimination lamp 11 paper cassette 13 fixing unit 14 discharge tray 51 transfer charger 52 separation charger 70 rotation fulcrum 71 blade holder 72 spring 73 cleaning blade

Continued on the front page F term (reference) 2H005 AA15 EA05 2H032 AA05 BA05 BA30 2H034 BF01 BF03 BF06 BF07

Claims (13)

[Claims] 1. A cleaning method in a cleaning device for use in an electrophotographic image forming apparatus, the cleaning method comprising: cleaning a surface of the toner image carrier provided in the image forming apparatus; A cleaning method in a cleaning device comprising at least one cleaning blade whose edge is pressed by a force of a pressing force f toward a surface, wherein the pressing force f of the cleaning blade (unit: g / m)
m) is a force applied per unit length of the cleaning blade, and fmin + S / {6 × D × (1-S)} ≦ f ≦ fm
a. A cleaning method characterized by having a value satisfying ax.
Here, S is a circularity, and S = (perimeter of a circle having an area equal to the area of the projected image of the toner particles) /
(Perimeter of the projected image of toner particles) and 0.95
≦ S ≦ 1 and D is the average particle diameter of the toner used (unit: μm)
Where fmin (unit: g / mm) is the minimum pressing force, and 1 ≦ fmin. Fmax (unit: g / mm) is the maximum pressing force, and 5 ≦ fmax ≦ 10. 2. The cleaning method according to claim 1, wherein the value of the minimum pressing force fmin is substantially 2 g / mm. 3. The cleaning method according to claim 1, wherein the value of the maximum pressing force fmax is further substantially 5 g / m.
m, the cleaning method in the cleaning device. 4. The cleaning method according to claim 1, wherein the cleaning blade is pressed against the toner image carrier in a counter direction. 5. The cleaning method according to claim 1, wherein said cleaning blade is made of polyurethane rubber. 6. The cleaning method according to claim 1, wherein said cleaning blade is pressed against said toner image carrier at a pressure contact angle of 3 to 25 degrees. Cleaning method in the device. 7. The cleaning method according to claim 1, wherein the cleaning blade is pressed against the toner image bearing member at a pressure contact angle of 5 to 15 degrees. Cleaning method in the device. 8. The cleaning method according to claim 1, wherein a free end length of the cleaning blade is 3 mm to 20 mm.
a length in the range of mm. 9. The cleaning method according to claim 1, wherein a free end length of the cleaning blade is 5 mm to 15 mm.
a length in the range of mm. 10. The cleaning method according to claim 1, wherein the cleaning device includes a second cleaning member in addition to the cleaning blade, wherein the second cleaning member includes a cleaning brush and a cleaning brush. A cleaning method in a cleaning device, which is one of a roller and a cleaning blade. 11. The cleaning method according to claim 1, wherein the toner image carrier is any one of a photosensitive drum, a transfer belt, and a transfer roller. Method. 12. A cleaning device for use in an electrophotographic image forming apparatus, the cleaning device comprising: a toner image carrier for cleaning a surface of a toner image carrier provided in the image forming apparatus; At least one cleaning blade whose edge is pressed by a force of a pressing force f toward the surface of the body, and a pressing force f (unit: g / m) of the cleaning blade;
m) is a force applied per unit length of the cleaning blade, and fmin + S / {6 × D × (1-S)} ≦ f ≦ fm
a cleaning device characterized by having a value satisfying ax.
Here, S is a circularity, and S = (perimeter of a circle having an area equal to the area of the projected image of the toner particles) /
(The perimeter of the projected image of the toner particles) and 0.95
≦ S ≦ 1 and D is the average particle diameter of the toner used (unit: μm)
Where fmin (unit: g / mm) is the minimum pressing force, and 1 ≦ fmin. Fmax (unit: g / mm) is the maximum pressing force, and 5 ≦ fmax ≦ 10. 13. An electrophotographic image forming apparatus provided with a cleaning device, wherein the cleaning device is provided on the surface of the toner image carrier to clean the surface of the toner image carrier provided in the image forming device. And at least one cleaning blade whose edge is pressed by the force of the pressing force f toward the pressing blade. The pressing force f (unit: g / m) of the cleaning blade is provided.
m) is a force applied per unit length of the cleaning blade, and fmin + S / {6 × D × (1-S)} ≦ f ≦ fm
An image forming apparatus comprising a cleaning device having a value satisfying ax. Here, S is a circularity, and S = (perimeter of a circle having an area equal to the area of the projected image of the toner particles) / (perimeter of the projected image of the toner particles), and 0.95 ≦ S ≦ 1 and D is the average particle diameter of the toner used (unit: μm)
Where fmin (unit: g / mm) is the minimum pressing force, and 1 ≦ fmin. Fmax (unit: g / mm) is the maximum pressing force, and 5 ≦ fmax ≦ 10.